Shrink film has been used for years to encapsulate articles. The shrink film must be able to shrink sufficiently to provide a smooth consistent coating. Previously, shrink films have been made from polyolefins and polyolefin blends and used extensively in the food and packaging business to protect and preserve articles such as food. One problem with polyolefin and polyolefin film blends is the difficulty of printing on the film. For printing to be successful, the films must provide a surface which will accept printing. Additionally the films must have sufficient tensile modulus to withstand the rigors of the printing process. Some polyolefin films do not have the tensile strength to withstand gravure printing. Some polyolefin shrink films may be able to withstand gravure printing but may still exhibit poor performance when placed on the article to be encapsulated, e.g., a battery.
Polyvinyl chloride (PVC) films provide acceptable shrinkages of about 40% to 45%. However, PVC shrink films have a problem with heat stability. Often, after the shrink film has been formed, the film may be exposed to elevated temperatures, such as in transport, which may cause the film to shrink prematurely. Another problem with PVC shrink films is concern over the environmental impact of PVC film, which forms harmful dioxins when incinerated. Concern regarding potentially adverse effect of halogens on the ozone layer has lead to efforts to provide halogen free shrink films.
Batteries are typically encapsulated by shrink films. The film must shrink sufficiently to encase the battery. A problem with encapsulating batteries and other cylindrical article is end puckering, which occurs when the shrink film does not shrink sufficiently to provide a smooth encapsulating film at the ends of the battery. The film folds over itself and forms a “pucker.” This puckering is unacceptable to consumers and, therefore, also to the manufacturer.
Battery encapsulating is done at very high speeds. The speed of the labeling is often more that 700 labels applied per minute. It is difficult for typical shrink film labels to work under such high speed conditions.